The nontuberculous mycobacteria (NTM) are ubiquitous environmental organisms foundin soil and water that rarely cause disease in humans. Since exposure to these organisms is universal and disease is rare, it can be concluded that normal host defenses are almost always sufficient to prevent infection. It follows that otherwise healthy individuals who develop disease must have abnormal susceptibility or immune defects that permit infection with nontubercuolous mycobacteria. The organisms that are most commonly encountered in clinical practice include Mycobacterium avium, and M. intracellulare [collectively known as the M. avium complex (MAC)], M. kansasii, M. fortuitum, M. abscessus, and M. chelonae. These organisms share significant structural and biochemical similarities with their more pathogenic relative, M. tuberculosis (MTB). Recognition of host factors that predispose or lead to NTM infection may have important implications for pathogenesis and therapeutic intervention, and may be applicable to the more virulent MTB. Identification of genetic or acquired susceptibility factors may lead to recognition of endogenous pathways that can be exploited therapeutically and to possible gene identification.